1. Field of the Invention
The present invention relates to an image forming apparatus which uses an electrostatic image recording method, an electrophotographic image recording method or the like. More particularly, the invention relates to an image forming apparatus which includes a plurality of image forming means, and performs image formation using an intermediate transfer member or a transfer-material bearing member.
2. Description of the Related Art
FIG. 7 is a cross-sectional view illustrating a principal portion of a conventional image forming apparatus.
An image forming apparatus 1P includes an image forming unit 10 including laterally arranged stations a, b, c and d each having substantially the same configuration, a feeding unit 20, an intermediate transfer unit 30, a fixing unit 40, and a control unit (not shown).
The respective units will now be described in detail.
The image forming unit 10 includes photosensitive drums 11a, 11b, 11c and 11d, each serving as an image bearing member. Each of the photosensitive drums 11a-11d is supported for rotation around its axis, and is rotatably driven in the direction of an arrow. Primary chargers 12a, 12b, 12c and 12d, optical systems 13a, 13b, 13c and 13d, and developing units 14a, 14b, 14c and 14d are disposed around the outer circumferences of the photosensitive drums, 11a, 11b, 11c and 11d in the direction of rotation, respectively. The primary chargers 12a-12d provide a uniform amount of charges on the surfaces of the photosensitive drums 11a-11d, respectively. The optical systems 13a-13d form electrostatic latent images on the photosensitive drums 11a-11d by exposing the photosensitive drums 11a-11d, respectively, using a light beam, such as a laser beam or the like, modulated in accordance with a recording image signal. Then, the electrostatic latent images are developed by the developing units 14a-14d which accommodate developers (toners) having four colors, i.e., yellow, cyan magenta and black, respectively. Cleaning devices 15a, 15b, 15c and 15d are disposed at portions downstream from image primary transfer regions Ta, Tb, Tc and Td for transferring the developed images onto an intermediate transfer member (an intermediate transfer belt 31, to be described later), respectively. Toner particles remaining on the photosensitive drums 11a-11d (those not transferred onto the intermediate transfer member 31) are scraped off by the cleaning devices 15a-15d, to clean the surfaces of the photosensitive drums 11a-11d, respectively.
According to the above-described process, image formation by the toners having the respective colors is performed.
The feeding unit 20 includes a cassette 21a and a manual feed tray 27 for accommodating sheets of a recording material P, pickup rollers 22 and 26 for individually feeding sheets of the recording material P from the cassette 21a and the manual feed tray 27, respectively, pairs of rollers 23 and a feeding guide 24 for feeding a sheet of the recording material P fed from the corresponding one of the pickup rollers 22 and 26 to registration rollers 25a and 25b, and the registration rollers 25a and 25b for conveying the sheet of the recording material P to a secondary transfer region in synchronization with the timing of image formation by the image forming unit 10.
The intermediate transfer unit 30 includes an intermediate transfer belt 31 made of PET (polyethylene terephthalate), PVdF (polyvinylidene fluoride) or the like. The intermediate transfer belt 31 moves around a driving roller 32 for transmitting a driving force to the intermediate transfer belt 31, a tension roller 33 for providing the intermediate transfer belt 31 with an appropriate tension by urging it by means of a spring or the like, and a driven roller 34 facing a secondary transfer region Te via the intermediate transfer belt 31. A primary transfer plane A is formed between the tension roller 33 and the driving roller 32. The driving roller 32 comprises a metal roller having a urethane-rubber or chloroprene-rubber coating a few mm thick on a surface thereof, in order to prevent slip relative to the intermediate transfer belt 31. The driving roller 32 is rotatably driven by a pulse motor (not shown). Primary transfer devices 35a-35d are provided at the primary transfer regions Ta-Td where the photosensitive drums 11a-11d face the intermediate transfer belt 31, respectively, at positions behind the intermediate transfer belt 31. A secondary transfer device 36 is provided at a secondary transfer region Te where the driven roller 34 faces the intermediate transfer belt 31, so as to face the driven roller 34 via the intermediate transfer belt 31. A cleaning device 50 for cleaning the image forming surface of the intermediate transfer belt 31 is provided on the image transfer belt 31 at a portion downstream from the secondary transfer region Te. The cleaning device 50 includes a cleaner blade 51 made of polyurethane rubber or the like, and a waste-toner box 52 for accommodating waste toner.
The fixing unit 40 includes a fixing roller 41a incorporating a heat source, such as a halogen-lamp heater or the like, a roller 41b pressed against the fixing roller 41a, a guide 43 for guiding the sheet of the recording material P to a nip portion constituted by a pair of the rollers 41a and 41b, internal discharging rollers 44 for guiding the sheet of the recording material P discharged from the nip portion further to the outside of the apparatus, and external discharging rollers 45. Reference numeral 46 represents a casing.
Next, the operation of the above-described image forming apparatus will now be described.
When an image-forming-operation start signal is provided, sheets of the recording material P are individually fed from the cassette 21a by the pickup roller 22. A sheet of the recording material P thus fed is conveyed to the registration rollers 25a and 25b after passing through the feeding guide 24 by the pair of feeding rollers 23. At that time, the registration rollers 25a and 25b are stopped, and the leading edge of the sheet of the recording material P contacts the nip portion. Thereafter, the registration rollers 25a and 25b start to rotate at the timing at which the image forming unit 10 starts image formation. The start of rotation of the registration rollers 25a and 25b is set to coincide with arrival of the sheet of the recording material P and the toner image being subjected to primary transfer on the intermediate transfer belt 31 in the image forming unit 10 at the secondary transfer region Te.
In the image forming unit 10, when an image-forming-operation signal is provided, the toner image formed on the photosensitive drum 11d at the most upper-stream portion in the direction of rotation of the intermediate transfer belt 31 according to the above-described process is subjected to primary transfer at the primary transfer region Td by the primary transfer device 35d where a high voltage is applied. The toner image subjected to primary transfer is conveyed to the subsequent primary transfer region Tc. At the primary transfer region Tc, image formation is performed while being delayed by the time of conveyance of the toner image between the adjacent image forming units from the preceding image formation at the primary transfer region Td, and the subsequent toner image is transferred in a state of being adjusted to the preceding toner image. Toner images of the four colors are transferred onto the intermediate transfer belt 31 in a state of being adjusted with one another by repeating the above-described process, and the resultant four-color image subjected primary transfer proceeds to the secondary transfer region Te.
When the sheet of the recording material P enters the secondary transfer region Te in synchronization with the arrival of the four-color toner image at the secondary transfer region Te and contacts the intermediate transfer belt 31, a high voltage is applied to the secondary transfer device 36 at the timing of the passage of the sheet of the recording material P through the secondary transfer region Te, and the four-color toner image formed on the intermediate transfer belt 31 according to the above-described process is transferred onto the sheet of the recording material P. Then, the toner image is fixed on the surface of the sheet of the recording material P by the heat and the nip pressure of the pair of rollers 41a and 41b. Then, the sheet of the recording material P is conveyed by the internal discharging rollers 44 and the external discharging rollers 45 to an external discharge tray 48 of the apparatus.
In the image forming apparatus using such a conventional intermediate transfer belt, the primary transfer surfaces of the respective colors are positioned on the same plane, and the plurality of image forming units are disposed so as to correspond to the primary transfer surfaces. Furthermore, a useless space is present within a unit where the intermediate transfer belt is stretched so as to provide the primary transfer surfaces of the respective colors on the same plane.
In the image forming apparatus using the conventional intermediate transfer belt, it is difficult to reduce the size of the overall apparatus due to the configuration of the apparatus.
Japanese Patent Application Laid-Open (Kokai) No. 5-72867 (1993) discloses a configuration for reducing the size of an image forming apparatus. In this configuration, four image forming means are separated as two upper and lower means. An intermediate transfer belt facing these means is folded at an acute angle after passing through the two upper image forming means, so as to face the two lower image forming means. According to this configuration, it is possible to reduce the area required for the intermediate transfer belt compared with the configuration of laterally arranging four image forming means as shown in FIG. 4. In the configuration of Japanese Patent Application Laid-Open (Kokai) No. 5-72867 (1993), when folding the intermediate transfer belt at an acute angle, the contact area between a driving roller present at the inner surface of a folded portion of the intermediate transfer belt and the intermediate transfer belt becomes large. When the contact area is large, the offset force in the longitudinal direction of the intermediate transfer belt becomes large, thereby increasing the amount of shift of the intermediate transfer belt. In this configuration, the folded portion is present after performing transfer operations at first two image forming units. Hence, if the amount of shift of the intermediate transfer belt is large, deviation from the image transfer position by the second two image forming units after the folded portion becomes large, thereby providing large deviation between images so as to degrade the picture quality. Accordingly, compatibility of reduction in the size of the apparatus and picture quality is difficult to realize.
In the conventional apparatus, when the intermediate transfer belt and the image forming units are disposed above the fixing unit, the intermediate transfer belt and the image forming units are directly influenced by the heat of the fixing unit so as to increase the temperature within the image forming apparatus, thereby degrading the intermediate transfer belt and developers (toner) within the developing units provided in the corresponding image forming units.
When an image forming unit of a black color is disposed at the most upstream portion of the intermediate transfer belt in the moving direction, the time required for providing a first printout of a black monochromatic color, which is most frequently used, increases.
The present invention has been made in consideration of the above-described problems. It is an object of the present invention to allow reduction of the size of an image forming apparatus which uses an intermediate transfer belt or a transfer-material bearing belt, without degrading the picture quality.
According to one aspect, the present invention which achieves the above-described object relates to an image forming apparatus including a plurality of image bearing members, each for bearing an image, a transfer belt onto which an image on each of the image bearing members is transferred, and transfer means for transferring the image on the transfer belt onto a transfer material. After images on the plurality of image bearing members have been sequentially transferred onto the transfer belt in a superposed state, the superposed images simultaneously are transferred onto the transfer material. An angle made by a first surface plane of the transfer belt which at least one of the plurality of image bearing members faces and a second surface plane of the transfer belt which image bearing members other than the at least one image bearing member face is substantially a right angle.
According to another aspect, the present invention which achieves the above-described object relates to an image forming apparatus including a plurality of image bearing members, each for bearing an image, a transfer belt onto which an image on each of the image bearing members is transferred, and transfer means for transferring the image on the transfer belt onto a transfer material. After images on the plurality of image bearing members have been sequentially transferred onto the transfer belt in a superposed state, the superposed images simultaneously are transferred onto the transfer material. An angle made by a first surface plane of the transfer belt which at least one of the plurality of image bearing members faces and a second surface plane of the transfer belt which image bearing members other than the at least one image bearing member face is substantially a right angle. The apparatus also includes a belt stretching member contacting an image transfer surface region of the transfer belt other than the first surface plane and the second surface plane, for stretching the transfer belt so that a contact portion between the belt stretching member and the transfer belt approaches the first surface and the second surface.
According to still another aspect, the present invention which achieves the above-described object relates to an image forming apparatus including a plurality of image bearing members, each for bearing an image, a transfer-material bearing belt for bearing and conveying a transfer material, and transfer means for sequentially transferring respective images on the plurality of image bearing members onto the transfer material born and conveyed on the transfer-material bearing member, in a superposed state. An angle made by a first surface plane of the transfer-material bearing belt which at least one of the plurality of image bearing members faces and a second surface plane of the transfer-material bearing member which image bearing members other than the at least one image bearing member face is substantially a right angle.
According to yet another aspect, the present invention which achieves the above-described object relates to an image forming apparatus including a plurality of image bearing members, each for bearing an image, a transfer-material bearing belt for bearing and conveying a transfer material, and transfer means for sequentially transferring respective images on the plurality of image bearing members onto the transfer material born and conveyed on the transfer-material bearing member in a superposed state. An angle made by a first surface plane of the transfer-material bearing belt which at least one of the plurality of image bearing members faces and a second surface plane of the transfer-material bearing member which image bearing members other than the at least one image bearing member face is substantially a right angle. The apparatus also includes a belt stretching member contacting a transfer-material bearing surface region of the transfer belt other than the first surface plane and the second surface plane, for stretching the transfer-material bearing belt so that a contact portion between the belt stretching member and the transfer-material bearing belt approaches the first surface plane and the second surface plane.
The foregoing and other objects, advantages and features of the present invention will become more apparent from the following detailed description of the preferred embodiments taken in conjunction with the accompanying drawings.